Is Hans Mikelson on either of the lists?  I have discovered a coding
problem in the lowpass version.  Looks like a failed optimisation in
csound4 but I do not know which is correct.

Or I guess I could look at csound4 code or csound3?

I have a version that checks for unstable values but I am not sure i
have the maths right yet.

Anyway it would be good to check with Hans